Wellbores may be drilled into earth formations to provide for location and production of various types of hydrocarbons. To form a wellbore, a downhole drilling tool with an attached bit at one end is advanced into the earth formation. As the drilling tool is advanced, a drilling mud or drilling fluid is pumped into the drilling tool and out the through the drill bit to provide for cooling of the drilling tool and carrying away of cuttings made by the interaction of the drill bit with the earth formation. In the drilling process, after interacting with the drilling tool, the drilling mud/fluid flows up through the wellbore to the surface. At the surface, the drilling mud/fluid may be collected and recirculated through the drill tool. In the process of drilling the wellbore, the drilling mud forms a mudcake/filter cake on the wall of the wellbore that may act to separate the wellbore from the surrounding earth formation.
During the drilling of the wellbore and/or after drilling of the wellbore, it is often desirable to evaluate the earth formations penetrated by the wellbore. In some processes, the drilling tool may be provided with devices to test and/or sample the surrounding formation in processes often referred to as measurement while drilling. In other processes, the drilling tool may be removed from the wellbore and a wireline with one or more attached tools may be deployed into the wellbore to test and/or sample the earth formations adjacent to the wellbore. In yet other processes, the drilling tool itself may be used to perform the testing or sampling of the surrounding earth formations. The testing and sampling of the earth formations may provide for formation evaluation, such as locating hydrocarbons, determining the presence of non-hydrocarbon fluids, determining a composition of formation fluids present in an adjacent earth formation and/or the like.
In a formation evaluation process, it is often necessary to draw formation fluids from the formation into a downhole tool for testing and/or sampling. Various devices, such as probes or the like, may be extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and provide for drawing formation fluid from the formation into the downhole tool. Such a probe for formation sampling may be a circular element that may be extended from the downhole tool and contacted with and/or pushed into/through the sidewall of the wellbore. A rubber packer may be provided at the end of the probe to provide for sealing the probe with the sidewall of the wellbore. Another device that may be used to form a seal with the wellbore sidewall is commonly referred to as a dual packer. In a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The mudcake/filter cake lining the wellbore may be useful in assisting the probe, dual packers or the like in making the seal with the wellbore sidewall. Once the seal is made, fluid from the formation may be drawn into the downhole tool through an inlet by lowering the pressure in the downhole tool. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. Patent Application No. 2004/0000433.
In the petroleum exploration and recovery industries, samples of formation fluids may be collected and analyzed for various purposes, such as to determine the existence, composition and producibility of subsurface hydrocarbon fluid reservoirs and/or the like. This aspect of the exploration and recovery process may be very important in developing drilling strategies and impacts significant financial expenditures and savings.
To conduct a valid fluid analysis, the fluid obtained from the subsurface formation should possess sufficient purity, or be virgin fluid, to adequately represent the fluid contained in the formation. As used herein, and in the other sections of this patent, the terms “virgin fluid”, “acceptable virgin fluid” and variations thereof mean subsurface fluid that is pure, pristine, connate, uncontaminated or otherwise considered in the fluid sampling and analysis field to be sufficiently or acceptably representative of a given formation for valid hydrocarbon sampling and/or evaluation.
Challenges/issues may arise in the process of obtaining virgin fluid from subsurface formations with regard to accessing the formation fluids to be sampled/evaluated. With regard to the petroleum-related industries, the earth around the borehole from which fluid samples are sought typically contains contaminates, such as filtrate from the mud/fluids used in the drilling process. This material may contaminate the formation fluid as the mud/fluid passes through the borehole, resulting in a combination fluid that is not the same as the virgin formation fluid and is, therefore, not useful for the fluid sampling and/or evaluation processes. Such a combination of drilling and formation fluids may be referred to herein as “contaminated fluid” or the like. Since in order to sample formation fluid from areas surrounding the wellbore, the samples must be sampled through the wellbore and the mudcake, cement and/or other layers comprising/surrounding the wellbore sidewall, it is difficult to avoid contamination of the fluid sample as it flows from the formation and into a downhole tool during sampling.
Various methods and devices have been proposed for obtaining pure formation fluids for sampling and evaluation. For example, U.S. Pat. No. 6,230,557 to Ciglenec et al., U.S. Pat. No. 6,223,822 to Jones, U.S. Pat. No. 4,416,152 to Wilson, U.S. Pat. No. 3,611,799 to Davis and International Pat. App. Pub. No. WO 96/30628 describe, among other things, sampling probes and techniques for improving formation fluid sampling. Additionally, guarded probes, such as disclosed in U.S. Pat. No. 6,301,959 to Hrametz et al., have been disclosed for formation fluid sampling. In a guarded probe, a sampling probe is provided that comprises two hydraulic lines to recover formation fluids from two zones in the wellbore. In operation, wellbore fluids—such as drilling mud, drilling fluids, filtrates of the foregoing or the like—may be preferentially drawn into a guard zone, connected to one of the hydraulic lines, while formation fluids may be drawn into a probe zone, connected to the other hydraulic line. Thus, the probe zone may collect purer formation fluids for analysis. However, while guarded probes may provide for better sampling, they are in general expensive and more complicated to effectively operate then a nonguarded probe.